The general forms of most metal detectors are either hand-held battery-operated units, conveyor-mounted units, or Vehicle-mourned units. Examples of hand-held products include detectors used to locate gold, explosive land mines or ordnance, coins and treasure. Examples of conveyor-mourned units include fine gold detectors in ore mining operations, and an example of a vehicle-mounted unit includes a unit to locate buried land mines.
These metal detectors usually, but not necessarily, consist of transmit electronics generating a repeating transmit signal cycle of a fundamental period, which is applied to an inductor, for example a transmit coil, which transmits a resulting varying magnetic field, sometimes referred to as a transmit magnetic field.
These metal detectors may also contain receive electronics that processes a receive signal from a measured receive magnetic field, during one or more receive periods during the repeating transmit signal cycle and/or at one or more frequencies to produce an indicator output signal, the indicator output signal at least indicating the presence of at least a metal target within the influence of the transmit magnetic field.
During the processing of the receive signal, the receive signal is either sampled, or synchronous demodulated, to produce one or more target channels, the one or more target channels being further processed to produce the indicator output signal.
Time domain metal detectors typically include pulse-induction (“PI”) or pulse-induction like metal detectors, and rectangular current-pulse metal detectors, wherein the receive processing includes sampling of the receive signal and/or synchronous demodulation over selected periods, which may include gain weighting of the samples or synchronous demodulation periods.
Frequency domain metal detectors typically include single or multi-frequency transmission, or pulse transmission with either sine-wave weighted synchronous demodulation, or unweighted synchronous demodulation with pre synchronous demodulation band-pass and/or low-pass filtering.
It is known to demodulate receive signals of a metal detector for processing to characterise the receive signals in terms of time constants. WO/2011/011820 discloses that discrimination of a target to a certain degree can be achieved by analysing the spread of the time constants. Mu-Hsin Wei, Scott, W. R., McClellan, J. H., “Estimation and application of discrete spectrum of relaxations for electromagnetic induction responses,” Geoscience and Remote Sensing Symposium, 2009 IEEE International, IGARSS 2009, vol. 2, no pp. II-105, II-108, 12-17 Jul. 2009 discloses that by identifying each of the main frequency components (inversely proportional to time constant) of the received signals, it is possible to enhance discrimination. Mu-Hsin Wei, Scott, W. R., McClellan, J. H., “Landmine detection using the discrete spectrum of relaxation frequencies,” Geoscience and Remote Sensing Symposium (IGARSS), 2011 IEEE International, vol., no., pp. 834, 837, 24-29 Jul. 2011 discloses a pre-screener to ignore cases where substantial soil signals are present.
The invention disclosed herein offers an alternative to the prior art for target discrimination.